In fusion reactors, there are essentially three ways in which energy may be transferred from the interior of a magnetically confined toroidal plasma column to the walls of the vacuum container, which is referred to in the art as the discharge tube. The first way is by emission of radiation in the form of soft x-rays (bremsstrahlung) as a result of continuous collisions of the rapidly moving high-energy charged particle in the plasma. Under the operating conditions of a full-scale fusion reactor, roughly 5% of the total energy released would be in the form of such radiation. The second way is by the slow ongoing diffusion of energetic particles outwardly across the confining magnetic field toward the walls of the discharge tube. In the case of a full-scale fusion reactor utilizing an equal mixture of deuterium and tritium, some 15% of the released energy would be carried away by these charged particles. The third way is by the emission of energetic thermonuclear reaction product neutrons, which would pass through the walls of the discharge tube and have to be captured in the region outside the discharge tube in a shielding blanket. About 80% of the released energy would be carried off in this form in a full scale fusion reactor.
Spitzer recognized that in a full-scale fusion reactor of the stellarator type, the walls of the discharge tube would be unable to withstand the energy impinging upon them from both the bremsstrahlung radiation and the outward-diffusing energetic particles. Accordingly, he modified the stellarator so that the outwardly diffusing particles would be prevented from reaching the main walls of the discharge tube, and be forced instead to dissipate their energy elsewhere. This modification, known as a toroidal "divertor", is shown in FIG. 4 of U.S. Pat. No. 2,910,414, and in FIG. 2 of U.S. Pat. No. 3,016,341.
The toroidal divertor was conceived as a basic component of a stellarator for confining a toroidal plasma column in a discharge tube forming a vacuum chamber, wherein the plasma was confined by an axial magnetic field coil around the discharge tube along an endless equilibrium axis. To this end, the axial field coil, which is also referred to in the art as a toroidal field coil for producing a toroidal magnetic field, produces toroidal field lines that were twisted with a rotational transform to provide concentric nested magnetic surfaces forming concentric shells of magnetic flux. Helical coils around the toroidal magnetic field provided the rotational transform and shear for stabilizing the stellarator plasma, as described in U.S. Pat. No. 3,002,912, the shear being defined herein (as in U.S. Pat. No. 3,607,627) as the rate at which the rotational transform changes in the nested set of co-axial surfaces from magnetic surface to magnetic surface, with r being the surface radius and L the length of the period over which the rotational transform is provided. Suitable helical stellarator coils, which are provided by a set of four parallel windings around the discharge tube having opposite currents in adjacent conductors and referred to the art as l=2 windings, are described in U.S. Pat. No. 3,278,384 (1966).
The basic principle of the toroidal divertor is that by the use of a reversed magnetic field, the thin outer cylindrical shell of magnetic flux, near the discharge tube wall, is brought out locally from the main discharge tube and spread out into a wider chamber. Charged particles diffusing outwardly from the body of the plasma enter the magnetic flux shell and follow the lines of force into the chamber where they strike collector plates to which they transfer their energy as heat. In essence, the effect of the divertor is to surround the main toroidal plasma column by a protective sheath or scrape-off layer that leads to an auxiliary divertor chamber where heat can be removed and the resulting cooled products of the thermonuclear reaction, as well as impurities, can be pumped off. The latter feature of pumping impurities is significant for both full scale and research reactors, since the impurities emitted from the walls of the discharge tube can have a deleterious effect on the plasma temperature, and/or confinement.
The heretofore known toroidal divertors comprised an annular ring forming a cylindrically symmetrical divertor chamber encircling the outside of the toroidal discharge tube cross-section and connected to the vacuum chamber therein through a hole on the inside diameter of the ring, such as shown in FIG. 8.13 of "Controlled Thermonuclear Reactions" by Glasstone and Lovberg, Van Nostrand, 1960. The diversion of the outer magnetic flux shell into the divertor chamber was achieved by means of a divertor coil in which flowed the same current that produced the plasma confining axial magnetic field, but in the opposite direction. By means of a vacuum pump, the particles diverted into the chamber through the hole on the inside diameter of the divertor ring were continuously removed. However, these toroidal divertors were limited to stellarators, comprised ring-shaped coils and chambers encircling the outside of the toroidal discharge tube cross-section co-axial with the tube axis and normal to the plane thereof, such that the coil and chamber ring encircled a small cross-section of the plasma column. Additionally, the toroidal divertor rings known heretofore made it difficult to assemble a suitable neutron absorbing shielding blanket along the entire plasma column ring cross-section. It is also advantageous to provide a poloidal divertor in a tokamak, where the toroidal plasma column is stabilized by the combination of an axial magnetic field and a plasma current for producing nested toroidal magnetic flux surfaces by providing a field component that twists the axial magnetic field lines into helixes around the equilibrium axis and produces shear. One such tokamak is described in U.S. Pat. No. 3,663,361.
It is still further advantageous to provide poloidal coils for producing gently curving field lines for vertical and radial stability, an initial plasma current, to remove the divertor coils from a separatrix around a toroidal magnetic field, and to provide a stagnation point on the inside diameter of a toroidal plasma column ring. As is well known in the art, a separatrix is defined as a magnetic flux surface in a set of nested magnetic surfaces, such as provided in a sheared toroidal magnetic field, outside of which the magnetic surfaces have a different shape from those inside. The stagnation point is the place from which the separatrix takes on its different shape, as described in U.S. Pat. No. 3,607,627, and "Plasma Physics," International Atomic Energy Agency, Vienna, 1965, page 391. Such flux surfaces, separatrices and stagnation points are described in U.S. Pat. No. 3,607,627.